Canned motor pumps use the pressure differential created by the pump impeller to drive fluid through the motor to lubricate the motor bearings as well as remove heat which is generated due to the inefficiency of the motor. In order to increase the useful life of the motor bearings and minimize erosion of the motor components which contact the cooling and lubricating fluid, it is preferable that the fluid be free of abrasives. Thus, the corrosive properties of the fluid, the fluid viscosity, and the vapor pressure characteristics of the fluid each must be considered when assessing the suitability of a fluid as a bearing lubricant and heat transfer medium.
In many applications, the fluid to be displaced by the pump is an abrasive slurry which is unsuitable for service in the motor. Accordingly, a clean, process compatible fluid is flushed through the motor from an external source. Clean fluid is flushed through the motor at a pressure determined by the input flow rate. Additionally, an auxiliary circulating impeller is seated on the motor shaft to facilitate the circulation of clean fluid through the motor housing.
An internal seal is provided between the motor and the pump to prevent the pumped slurry from entering the motor housing and contacting the motor components. Preferably, the externally supplied fluid pressure is greater than the pressure of the slurry immediately upstream of the seal, so that clean motor fluid at high pressure leaks across the seal and into the pump housing. The restriction provided by the internal seal accelerates the leakage flow to oppose backflow of the slurry into the motor. The motor fluid which leaks into the pump housing, sometimes called barrier fluid, is dispelled by the pump impeller and discharged through the pump outlet.
One problem which exists with the above construction is that the clean fluid which is flushed through the motor and leaks across the internal seal must be distilled from the mixture which is discharged from the pump. Distillation of the motor fluid and the slurry is an expensive, time consuming process which can greatly limit the usefulness of the canned motor pump.
Another problem is that because leakage through the internal seal is directly proportional to the pressure differential across the seal, in applications where the pump-side pressure is relatively low, minimum leakage requirements and the necessary small pressure differential across the seal limit the pressure which can be maintained in the motor.
The maintenance of high fluid pressure in the motor is desired because when a volatile fluid is used to cool and lubricate the motor, temperature rises in the motor can cause the fluid to vaporize and cause extensive motor damage via cavitation effects or by blocking the flow. It therefore is desirable to maintain the motor fluid at a pressure which is greater than the fluid's vapor pressure to maintain the fluid in the liquid phase. However, fluid pressure in the motor housing is equal to the pump-side reference pressure at the seal plus the pressure differential across the seal. The capability of maintaining a high motor pressure therefore requires either (a) a large pressure differential across the seal, or (b) an increased pump-side reference pressure of the slurry.
For instance, if the slurry pressure at the pump-side of the seal is 10 psi, in order to minimize the leakage through the seal, it is necessary to maintain a relatively low differential pressure across the seal and hold the fluid pressure in the motor housing at approximately 12 psi. However, when the vapor pressure of the externally supplied fluid in the motor housing is 20 psi, the fluid pressure in the motor housing must be increased above 20 psi to reduce the risk of motor fluid vaporization.
A relatively high motor pressure could be accommodated together with a low pump reference pressure by increasing the pressure differential across the seal As discussed above, however, an increased pressure differential causes increased leakage through the seal and results in the necessity of distilling motor fluid from the particular slurry which is pumped.
Alternatively, increased motor pressure could be provided together with a pressure differential which meets minimum leakage requirements by increasing the pump-side pressure of the slurry. For instance, if the slurry pressure on the pump-side of the seal was 19 psi, the motor pressure could be raised above the motor fluid vapor pressure and a low pressure differential could be maintained across the seal. Pump-side reference pressure is increased by shrouding the pump impeller or by adding vanes to fixed structure adjacent the impeller. The problem with this solution is that increasing the reference pressure in the pump housing increases the thrust force acting against the backside of the impeller, which degrades axial impeller balance and shortens the service life of the impeller thrust bearings.
The foregoing presupposes that leakage across the internal seal is unavoidable. Although present axial face seals are capable of essentially preventing any leakage, such seals provide limited utility in canned motor pump applications. In certain applications, canned motor pumps use a liberal axial float design in which the motor shaft preferably is free to move axially in order to accommodate manufacturing tolerances and axial wear of the thrust bearings. Because axial face seals are dependent upon precise axial positioning of the shaft they inherently restrict the versatility of an axial float design. In addition, face seals generally require precisely machined faces to maintain a sufficiently flat surface for achieving proper sealing engagement and an associated mechanism for continuously biasing the flat surface into sealing engagement as the surface wears. As a result, face seals are complex and expensive devices which detrimentally impact the performance and cost of a canned motor pump.
Conventional radial bushing seals require tight radial clearances but which are not dependent upon precise axial positioning of the shaft. Radial bushing seals also are less expensive and mechanically simpler than axial face seals. Unfortunately, present canned motor pumps are unable to take full advantage of the benefits of radial bushing seals.
Although radial bushing seals accommodate an axial float design and are more easily and inexpensively manufactured than face seals, radial bushing seals do not provide as complete a seal as face seals and may not satisfy minimum leakage requirements at the differential pressures required. Because the leakage flow rate across the bushing seal is a function of the differential pressure across the seal, it is preferable to hold a low differential pressure across the seal to maintain a minimum seal flow rate. For the reasons discussed above, this characteristic is a problem when the motor pressure required to hold a minimum seal flow rate is lower than the vapor pressure of the fluid.